Our program is focused on the interrelationships of cell structure and gene expression that mediate osteoblast proliferation and differentiation during progressive development of the bone cell phenotype. Our working hypothesis is that nuclear structure (the nuclear matrix, chromatin structure, matrix-chromatin interactions, and higher order chromatin functional domains) contributes to the onset, progression and maintenance of osteoblast phenotypic properties by participating in the regulation of gene expression both at the transcriptional level and at a series of post-transcriptional levels. Equally important, we are postulating that nuclear structure is regulated by the expression of genes related to phenotypic properties of the bone cell. We will pursue an integrated team approach to experimentally address molecular mechanisms whereby nuclear structure regulates developmental expression of cell growth and bone cell-related genes in normal diploid osteoblasts. Mechanisms by which cell structure facilitates the integration of physiological regulatory signals (1,25(OH)2D3, dexamethasone and TGFBeta) that support osteoblast growth and differentiation will be explored. Parallel studies will be carried out with osteosarcoma cells where stringent growth control is abrogated, an organized extracellular matrix is not developed and genes associated with expression of the mature osteoblast phenotype, which in normal diploid osteoblasts are expressed post-proliferatively, are actively expressed in proliferating osteosarcoma cells. The relationship of cell structure to the expression of genes required for osteoblast proliferation and differentiation will provide valuable insight into the cellular molecular basis of both normal skeletal development and metabolic bone disease.